In this description of the invention, certain terms have the meanings ascribed to them. "Fiber" or "fibers" refers to either staple length fibers or continuous filaments. "Bicomponent" refers to a fiber cross-section where two different polymers are disposed in a longitudinally coextensive relationship. e.g., sheath-core, side-by-side, islands-in-sea. "Conductivity" refers to the characteristic exhibited by staple fibers and continuous filaments which dissipate electrostatic charges. For the purposes of the present discussion, resistives up to 10.sup.10 ohms/cm and preferably 10.sup.8 -10.sup.9 ohms/cm are considered indicative of conductive fibers.
It is known that friction generates static electricity in synthetic fibers, such as polyamide fibers, polyester fibers, acrylic fibers, etc., and also in some natural fibers like wool. This is a disadvantage of synthetic fibers, especially when such fibers are used in applications where the discharge of static electricity (the characteristic shock) can have serious consequences. For example, the discharge of static electricity can damage computers and other electronic equipment. In some cases, such as in flammable atmospheres, the discharge of static electricity can result in a fire or explosion.
Because of the propensity of certain fibers to generate (or not dissipate) an electrical charge and because fibers are prevalent in many environments where static electricity is undesirable (carpet in computer rooms, clean room garments, etc.) a large number of proposals to address the generation of static electricity have arisen. In general, these methods concern either imparting conductivity to the fibers themselves or to the article made from the fibers by incorporating one or more individually conductive fibers in the article or treating the fibers or article made from fibers with an antistatic surface treatment. Surface treatments are not generally desirable.
The invention concerns conductive fibers for incorporation into fibrous articles like carpet or textiles. One of the proposals is to mix electrically conductive carbon back in the synthetic fibers. There exist a variety of fiber cross-sections where a portion of the cross-section contains carbon black (or some other conductive material like metal).
One cross-section involves penetrating carbon black or metal particles into the periphery of a synthetic fiber. This method has the disadvantage of being labor intensive and also requiring specialized equipment for handling the fiber during the penetration step. The fibers made by this method sometimes flake off the conductive layer adhered to the surface, requiring special handling to ensure that this does not happen.
U.S. Pat. No. 4,388,370 to Ellis et al. describes a drawn melt spun sheath-core bicomponent fiber where carbon black is penetrated into the periphery of the fiber. The sheath has a lower melting point than the core to facilitate the penetration of the carbon black (or finely divided metal).
U.S. Pat. No. 4,242,382 to Ellis et al. describes another process for adhering electrically conductive particles to the surface of a fiber. An article entitled Epitropic; ICI's Surface Modified Antistatic Fibre, Fibre Technology, Textile Month, August, 1993, pp. 40-41, describes a polyester bicomponent fiber with electrically conductive particles adhered to the surface.
Sheath-core bicomponent fibers with conductive sheaths have been made also by co-spinning the conductive composition with the non-conductive composition in an arrangement where the conductive composition forms a sheath around a core of the non-conductive composition. Such a bicomponent fiber for brush applications is described in U.S. Pat. No. 4,610,925 to Bond. Being designed for use in hairbrushes, the Bond fiber is very large (a diameter of at least 0.25 mm). Because the sheath and core are made of different polymers, this type of fiber also may tend to flake or defibrillate at the sheath-core interface.
Another cross-section is made by co-spinning a nonconductive material with a conductive material in a predetermined relationship to achieve a conductive core/non-conductive sheath relationship. Such a fiber is disclosed in U.S. Pat. No. 3,803,453 to Hull. The Hull fiber preferably is a bicomponent fiber. Hull acknowledges the relatively fragile nature of these fibers by teaching to exercise care in the drawing of them, e.g., avoiding sharp corners.
U.S. Pat. No. 4,085,182 to Kato describes a conductive core sheath-core bicomponent electrically conductive synthetic fiber made by simultaneously melt spinning the conductive and non-conductive compositions in a sheath-core arrangement and taking up the fibers at least 2,500 meters per minute. The "high speed" take-up is taught to make a drawing step unnecessary. The resistance of the Kato fiber is on the order of 10.sup.8 to 10.sup.9 ohms/cm.
However, fibers where the non-conductive portion completely covers the conductive portion suffer from generally decreased conductivity. One method of addressing the problem of decreased conductivity in a conductive core arrangement is to arrange the conductive materials and non-conductive materials in a fashion where the conductive material is partly exposed to the surface, for example, by offsetting the core. U.S. Pat. No. 4,216,264 to Naruse et al. describes a fiber having a carbon black containing electrically conductive section radiating from the core of the fiber and extending in at least two directions. The resistance of the fibers was less than 1.times.10.sup.13 ohm/cm (no less than 1.4.times.10.sup.8 per filament. The conductive sections and non-conductive sections are preferably made of the same polymer.
U.S. Pat. No. 4,756,969 to Takeda describes a fiber of a modified sheath-core type where the sheath includes layers of nonconductive material and electrically conductive material. The electrically conductive material is exposed at a fraction of the fiber's periphery.
U.S. Pat. No. 4,420,534 to Matsui et al. describes a bicomponent fiber having generally internal layers of conductive material. The fiber is made from two polymers differing in melting point by at least 30 degrees. Matsui recognizes the problem of lost conductivity caused by drawing fibers and proposes several methods to address the problem. One of these methods involves relaxing the drawn fiber at a temperature above the melting or softening point of the lower melting polymer but below the melting or softening point of the other polymer. The specific resistance of the Matsui fiber is 3.5.times.10.sup.3 ohms/cm or higher.
U.S. Pat. No. 4,129,677 to Boe describes a side-by-side bicomponent fiber where the conductive portion occupies a portion of the periphery of the fiber. The resistance of the Boe fibers is 1.89.times.10.sup.8 ohms/cm or higher.
U.S. Pat. No. 3,969,559 to Boe describes a side-by-side bicomponent fiber where the nonconductive constituent partially encapsulates the conductive constituent.
Controlling the degree that the conductive component is exposed to the fiber surface is difficult in production. For example, the conductive component might become excessively covered with the non-conductive component (sometimes the non-conductive component completely covers the conductive component) and the conductivity of the fiber consequently lowers. Also, the use of electrically conductive materials is known to affect the properties of the fibers, for example, the spinnability, strength and elongation are typically decreased. It remains a goal of the efforts to address static electricity in fibers by making an electrically conductive fiber to dissipate static and yet to process like and have the properties of regular (non-conductive) synthetic fibers.